Comparative growth rates and yields of ciliates and heterotrophic dinoflagellates

Growth rates, ingestion rates and grazer yields (grazer volume produced/prey volume consumed) were measured for six protozoan species (ciliates: Favella sp., Strombidinopsis acuminatum, Uronema sp.; heterotrophic dinoflagellates: Amphidinium sp., Gymnodinium sp., Noctiluca scintillans) in laboratory batch culture experiments. Comparative growth data indicate that the prymnesiophyte Isochrysis galbana, the prasinophyte Mantoniella squamata, two cryptophyte species and several autotrophic dinoflagellate species were suitable foods for these grazers. When grown on optimized diets at 13 degrees C, maximum ciliate growth rates (range 0.77-1.01 day(-1)) uniformly exceeded maximum heterotrophic dinoflagellate growth rates (range 0.41-0.48 day(-1)). A compilation of published data demonstrates that this growth rate difference persists across a range of ciliate and dinoflagellate taxa and cell sizes. Comparison of volume-specific ingestion rates and yields for the six species studied here showed that there was no single explanation for this growth rate disparity. Heterotrophic dinoflagellates exhibited both low ingestion rates and, in one case, low yields; ciliates were able to achieve higher growth rates via either higher ingestion rates or higher yields, depending on ciliate species. Volume yield increased over time throughout the exponential growth phase in nearly all experiments, suggesting variation in response to changing food concentrations or long-term acclimation to culture conditions. Higher maximum ciliate growth rates mean that these grazers have the potential to exercise tighter control over incipient blooms of their prey than do heterotrophic dinoflagellates.